I. Field of the Invention
This invention relates generally to fluid handling equipment, and more particularly to an improved, low-cost positive displacement pump especially designed for residential high-pressure washing applications.
II. Discussion of the Prior Art
Generally speaking, most municipalities deliver water to residential establishments at a pressure of about 60 psi. Such pressure is oftentimes insufficient for such applications as automobile washing, garage floor and driveway cleanup, removing the flaking paint from building sidings and similar applications. End-uses like this require that water be delivered at relatively high pressures and at acceptable flow rates such that the momentum of the water striking the surface to be cleaned will be effective to dislodge the dirt, grease and grime to be removed. It has been found that a pump delivering water at about 1,000 psi and at a flow rate of two gallons/minute is satisfactory for most residential pressure washing applications.
A wide variety of positive displacement pump devices are available which will meet such requirements. Single or multiple piston pumps, when driven by an electric motor or internal combustion engine can readily meet the pressure and flow rate requirements. However, such pumps, in the past, have tended to be relatively heavy, fairly complex and, therefore, relatively expensive, making it difficult to sell to average homeowners.
For example, applicant's assignee, Hypro Corporation of St. Paul, Minn., offers a line of single and of multiple piston positive displacement pumps of the type including a crankcase, a cylinder block affixed to the crankcase, a shaft journaled for rotation in the crankcase having one or more eccentrics and coupled by an appropriate number of connecting rods to the piston which is mounted for reciprocal movement within a cylinder bore formed in the cylinder block. Pumps of this type also require suitable inlet and outlet poppet valves as well as appropriate seals operatively disposed between the cylinder walls and the piston to prevent the liquid being pumped from entering the crankcase and for effectively isolating the low pressure or suction port from the high pressure discharge port.
To meet the average residential consumer demand for a relatively low cost, light-weight pump for high pressure washing applications around the home and farm, Hypro Corporation engaged the Southwest Research Institute in San Antonio, Tex., to develop a pump for home use which would meet prescribed and rather stringent specifications in terms of cost and performance. Rather than attempting to cost reduce existing pump designs developed for commercial and industrial applications so that the resulting product could be more readily marketed to the general consumer, it was decided instead to adapt a particular type of pump that heretofore had been used primarily as a metering pump, i.e., a pump for introducing a measured quantity of a liquid from a supply reservoir into a fluid stream, into one suitable for high pressure washing applications.
The prior art metering pump comprises a crankcase for journaling a driven shaft about a longitudinal axis and a cylinder block having a cylinder receiving bore formed therein with the cylinder block being affixed to the crankcase housing such that the axis of the driven shaft is at a predetermined angle relative to the longitudinal axis of the cylinder bore. A cylindrical piston is disposed in the cylinder bore and is affixed to the driven shaft by means of a connecting pin and a spherical bearing. Because of the angle at which the axis of the piston is out of alignment with the axis of the driven shaft, as the driven shaft is rotated, the piston both rotates and reciprocates within the cylinder. A suction port is formed through the cylinder block on one side surface thereof while the discharge port is located on a diametrically opposite surface of the cylinder block. The piston is appropriately formed to provide a pocket which sweeps past the suction port to thereby pick up a charge of water and then as the piston rotates toward the discharge port, the reciprocating motion of the piston increases the pressure on the fluid until the pocket becomes exposed to the discharge port. Such a pump design offers the advantage of obviating the need for inlet and outlet flow control valves.
Examples of the prior art rotating/reciprocating piston-type pump are described in U.S. Pat. Nos. 3,168,872; 2,517,645 and 1,244,160. Such pumps have typically been designed to operate at low flow rates and relatively low pressures which, while satisfactory for metering applications, are wholly insufficient for a portable, high pressure spray wash application. To achieve adequate flow rates for high pressure washing, it is desirable to drive the pump at relatively high RPM, e.g., about 10,000 rpm. Likewise, to achieve a pump discharge pressure of about 1,000 psi, a prime mover of at least 1.5 horsepower is dictated. Keeping in mind that size, weight and cost are important constraints, there are stringent limitations imposed on the pump's stroke volume and, hence, the need for operating at high rpm. The high discharge pressure desired coupled with the relatively small pump size and the inherent imbalance of forces acting on the piston as it rotates to alternately expose and block the suction and discharge ports presents a real problem in maintaining the piston centered within the cylinder so as to avoid scraping and galling of the walls of the cylinder bore and of the piston itself. Likewise, the unbalanced pressures makes it somewhat difficult to create seals which will hold up over long periods of use.
Another problem encountered when it is desired to operate a prior art pump of the type disclosed in the aforereferenced patents at high rpm rates is that of maintaining adequate lubrication of the bearing used to operatively couple the piston to the pump's driven shaft. The conventional mode of lubrication for a metering pump operated at relatively low rpm rates is to fill the crankcase housing with oil and then provide adequate seals between the cylinder wall and the piston to insure that the liquid being pumped does not flow into the crankcase to contaminate the oil. It is found that when this lubrication approach is attempted in a positive displacement pump of the type under consideration driven at 10,000 rpm, a significant loss in pump efficiency results due to the work involved in moving and churning the oil within its crankcase. Also, with the high pressures desired, the difficulty in providing an effective oil seal which will hold up over prolonged periods of use is difficult and expensive to attain. At high rpm and pressures typical of the present invention, pressure pulsations may cause problems because of excess stresses in the pump or attached hose and excess noise.